Catheters are used in a variety of medical applications, and in some, they must be precisely positioned initially and kept in that position despite movement of the patient or other factors One example of such catheters are tracheal tubes. With many medical patients, it is necessary to insure that the breathing passageway is kept open at all times, and this is accomplished by a tracheal tube. The tube is inserted through the patient's mouth or nose, and it extends down into the patient's throat and into the patient's windpipe or trachea. Air can then pass through the tube lumen into and out of the patient's lungs. The principal drawback of these tubes is that there is not much room for error in placement of the distal or internal end of the tube inside the patient s windpipe in order for the tracheal tube to be effective, particularly with a pediatric patient. In fact, the distal end of the tube must be kept at a relatively specific position, which is at about the midpoint of the trachea below the vocal cords and above the carina. If the tube is inserted too far down into the trachea, its distal end may extend into the bronchial tree for one lung, and thus the other lung will receive an inadequate supply of air and may collapse. On the other hand, if the distal end of the tube is not inserted far enough into the trachea, it may interfere with the vocal cords, or it may be accidentally extubated and enter the esophagus with the result that air will not reach the lungs
It is somewhat difficult to position a tracheal tube properly in the trachea, and in fact, tracheal tubes are usually inserted with the aid of a laryngoscope. Once properly in place, the tube is usually anchored by inflating a cuff on the distal end (for adult tracheal tubes) and taping the proximal or external end to the patient's skin. Nevertheless, even if properly positioned initially, and despite being secured in place, the tubes can and do move because of normal movement of the patient as well as during medical procedures such as surgery. Thus, the position of the distal end of the tube must be frequently monitored.
In the prior art, this monitoring is usually done by indirect means such as auscultation of the chest, which involves listening to breathing sounds for both lungs. It is also done by checking for chest motion, as well as generally observing the depth of insertion of the tube based on markings on the portion of the tube outside of the patient's mouth. These methods are generally unsatisfactory as they all really depend on a physiologic response of the patient as the basis for determining the position of the tube. The other common method for monitoring tube position is by X-ray. In addition to being expensive and exposing the patient to radiation, it also has the principal drawback that by the time the X-ray is taken, developed and returned, the tube may have moved again. Accordingly, the X-ray technique is unsuitable, particularly in critical care or emergency situations, when such tubes are commonly used.
As might be expected, the positioning and monitoring problems are particularly acute with infants or pediatric patients, who have such tubes inserted in most critical care situations and for up to three-fourths of all surgery involving general anesthesia. There is much less margin for placement error in infants or pediatric patients because the tracheas are much shorter. Also, the tracheal walls of infants and children are fragile and much more susceptible to mucosal wall irritation and rupture, which could be caused by any abrupt surface discontinuity on the tube. For this and other reasons, tracheal tubes for infants do not have inflatable cuffs to anchor the distal ends in place. Thus, they are more susceptible to movement. Consequently, proper tube placement and monitoring are much more difficult and critical with pediatric tracheal tubes.
A new method of and apparatus for accurately determining the positioning of a tracheal tube inside a trachea is disclosed in McCormick U.S. Pat. No. 4,431,005 and its related patents, Bresler U.S. Pat. No. 4,445,501, and Bresler U.S. Pat. No. 4,416,289, all of which are incorporated herein by reference. In these, the position of a tracheal tube inside the trachea is determined by a device which generates a narrow electromagnetic field and detects disturbances in that field caused by a piece of metal placed around the outside of the distal end of the tracheal tube. The metal disturbs the narrow field even through biological tissue. Accordingly, the position of the distal end can be determined when the device detects the resulting field disturbance. The tube for use with this device is shown in the patents. It has a piece of metal wrapped around and secured to the outside of a commonly used plastic tracheal tube. The metal is then covered by a plastic coating.
Unfortunately, these particular tubes are unsatisfactory, particularly for pediatric patients, for a number of reasons. First, even though it has a thin coating, the metal band can still irritate a child's trachea very easily because it protrudes significantly from the outer wall surface of the tube. Moreover, the metal band cannot be protected by an inflatable cuff. Such cuffs are unsuitable for use with the fragile wall of an infant's trachea, and they would also cause insertion problems due to the narrow space in which the tube must fit. Further, the pediatric tracheal tubes have narrow lumens and thin walls, making them both flexible and easily compressible. Thus, wrapping the metal band too tightly around the preformed tube, as might occur occasionally during manufacture, might lead to a compression of the wall and the critical air passageway in the tube. (The allowable tolerance for the inside diameter of such tubes is a mere .+-.0.003 inches.) On the other hand, failure to wrap the band tightly might result in serious consequences if the band should dislodge during use.
Finally, there is one additional drawback which applies to prior art catheters in general (not merely those with metal bands and the like). All tracheal tubes contain printed labeling on the outside surface of the tube. The labeling includes certain required medical nomenclature as well as depth of insertion markings. The latter is to allow medical personnel to check the depth of insertion of the tube by examining the depth markings on the exposed portion. Unfortunately, the ink used to print the markings may come off when it is rubbed in the presence of moisture or anesthetic gas. These conditions frequently exist when the tube is in use, and often the depth markings at the exposed end are erased or obscured as a result. When this occurs, the tube may have to be replaced. Further, while there are standards limiting the toxicity of the ink used, the ink is nonetheless toxic to some degree, and it would be preferable to prevent its contact with tissue.